603- Will It Hurt Anything To Use An Afterglow Relay With An Early Glow Plug Harness?

[Squiggle Dog]

Here's the deal--my friend has a 1991 W126 350SDL. As can be expected, the original 3.5 liter OM603 engine failed, so it was replaced with a 3.0 liter OM603 engine out of a 1987 W124 300D.

Most of the glow plugs weren't working, so I replaced all of them with new Bosch ones of the same length and part number. Even though the original plugs that came with the engine weren't afterglow rated, all new ones supposedly are.

But, the plugs were working intermittently and the relay was making noise, getting hot, and melted a bit. So, the relay is bad.

The previous owner removed the original afterglow-type relay from the W126 and installed the older-style relay from the W124 that doesn't have afterglow. This was a smart move since the old plugs weren't afterglow rated.

But, now since the car has afterglow rated plugs installed, I figure why not put the original afterglow-type relay in? But, I ran into a problem; the temperature sending unit on the head has only one prong, not two (the second one is for the afterglow feature). The glow plug harness was also taken from the W124, so it only has provisions for the one prong sending unit.

So, in order to restore the afterglow, I have to replace the sending unit in the head and replace the glow plug harness with the type that takes two prongs. But, this is going to cost a good bit of money and 350SDLs are never in the wrecking yards here.

My questions are:

1. Can I just put an afterglow-type relay back in the W126 as intended and have it not damage anything, even though the sending unit on the head and the glow plug wire harness are missing the afterglow wire provisions?

2. Will the relay still work, but I just won't have afterglow until I replace the sending unit and wire harness later?

3. Or would it be better to use an older style (W124) relay until I get the parts replaced?

The reason I ask is because my roommate has to buy a new relay and we don't know what type to get. I am thinking the afterglow type as original to the W126 is the best to use (it just won't afterglow), and then the sending unit and glow plug harness can be replaced at a later date to make the afterglow operational.

[Squiggle Dog]

Alternatively, there is a temperature sending unit on the side of the water pump housing that is not being used. I wonder if I could run a wire from that to the second port on the wire harness on the body that receives the glow plug harness.

Or, I suppose I could just make a jumper wire so the one prong at the end of the W124 glow plug harness becomes two like the W126 harness had. No reason that wouldn't work, right? Probably a cheap alternative to replacing the sending unit and harness?

[BrianCostello]

I would use the W124 603 non-afterglow relay. That will match the wires currently installed on your motor.

Then do the cheap and simply "afterglow" mod. There is a great write up on here, you simply wire in another relay . . says the guy who has the parts sitting on a shelf . . .

[VA-Merc]

I chose to add afterglow using the factory 350SDL relay to my 87 300D, it wasn't really that hard. I added a couple of wires, changed the sending unit behind the injection pump (I did not have to remove the injection pump as some have indicated is required), and then put in the afterglow relay (my original relay had finally failed after 270,000 miles). Voila...I had afterglow for about an hours worth of work.

I know some have mentioned it may be better to use the non-afterglow relay and add in a separate generic relay, and there is certainly nothing wrong with doing that. I personally liked the idea of the OE afterglow relay, especially once I realized I could pretty easily change the sending unit without moving much out of the way. I had to replace the relay anyways so I just ordered one for a newer W126.

[BrianCostello]

I believe the reason for using the non afterglow relay was the price.

Non after glow is $150-200 range and the afterglow relay was running $300 plus.

That is old information from memory. I could be wrong.

[Hit Man X]

I have a W140 after glow relay on my SDL's original harness, no problems after many miles. All I remember doing was swapping the relay and adding a wire to a sender. Sorry, it has been nearly 10 years since I did the modification.

Really smooths out the cold start of the 603

[Squiggle Dog]

I wonder if not having the wire to the sender connected would damage anything if the afterglow relay is used, but I am thinking not. The wire harness on the body has the spot for the second sender wire. It's too bad that the harness on the engine only has one sender wire.

It looks like the price for either relay is about the same. I'm thinking use the afterglow relay and just not have it afterglow, and then at a later point, replace the engine harness and sender to the two-prong type, or add a wire so that afterglow will be functional.

Then again, this is Phoenix and it never gets that cold here.

[Squiggle Dog]

Well, my roommate purchased an engine wire harness that takes the 2-prong sender and will fit into the body wire harness. So now he'll just need to get a 2-prong sender for the block and buy an afterglow relay and he'll be all set like it was from the factory--only with the short version glow plugs in an older OM603--but the plugs are afterglow rated and with the tapered tips, so it should work out.

[PaulH]

I have a '87 603 that starts well because I let the glow plugs on for 5-10 seconds after the dash light goes out.

[mike-81-240d]

I unhooked the purple wire. I now have full 30 second glow every time.

[Squiggle Dog]

I just ended up installing an original-type glow plug harness for two pins, a two-pin sensor, and the proper type relay with afterglow. So now the 3.5 car is as original except for the different length plugs in the 3.0 engine, which are fortunately now rated for afterglow.